Heretofore, it has been common practice to utilize either cylindrical tubes or tubes having an oblong shape with the narrow sides rounded extending between the top and bottom headers of a radiator. It has additionally been advocated in Bellovary et al U.S. Pat. No. 4,159,034 to use tubes having such oblong shaped bodies which are formed at the opposite ends thereof into cylindrical shapes for generally easier connection to the respective top and bottom headers. Robertson U.S. Pat. No. 2,105,267 illustrates an oval-to-cylindrical tube arrangement. It is important in such arrangements that the transition section from the flattened body to the cylindrical end be able to withstand stressing during assembly, and/or loading by of the header after assembly, without collapsing or bending and thus pinching off the flow area through the juncture between the body and the transition section.